Single-Capacitor Phase-Controlled Series Resonant Converter

A new phase-controlled series resonant dddc converter is described, analyzed, and experimentally verified. The circuit comprises a phase-controlled inverter and a Class D current-driven rectifier. The phase-controlled inverter consists of two switching legs, two resonant inductors, and a single resonant capacitor connected in series with an ac load. The phase shift between the voltages that drive the MOSFET’s is varied to control the ac current of the inverter and thereby regulate the dc output voltage of the converter. A frequency-domain analysis is used to derive basic equations which govern the circuit operation. An important advantage of the converter is that the operating frequency can be maintained constant. For operation at a switching frequency greater than 1.15 resonant frequency, the load of each switching leg is inductive. The proposed converter has an excellent full-load and part-load efficiency. An experimental prototype of the converter with a center-tapped rectifier was built and extensively tested at an output power of 78 W and a switching frequency of 200 kHz. The theoretical and experimental results were in good agreement. I . INTRODUCTION HASE-CONTROLLED series resonant inverters [ I]-[3] P and converters [4]-[7] can be operated at a fixed frequency, while conventional resonant converters [8]-[ 121 are controlled by varying the operating frequency, usually over a wide range. A variable operating frequency is a very undesirable feature because it is difficult to handle electromagnetic interference (EMI) and filtering problems and to effectively utilize magnetic components. In the phase-controlled inverters described in [3], [4], two switching legs are used to drive one resonant circuit. A phase shift between the pairs of the drive voltages of power switches is varied to regulate the dc output voltage. However, the resonant circuit represents an inductive load for one switching leg and a capacitive load for the other. For power MOSFET’s and IGBT’s, operation with inductive loads is preferred [ 111. For capacitive loads, current spikes are generated by reverse recovery of antiparallel diodes. Snubbers can alleviate this problem, but the converter circuit becomes complex. On the other hand, thyristors do not turn off naturally with inductive loads and therefore cannot be used as power switches in such converters. The singlecapacitor phase-controlled series resonant converter analyzed in this paper eliminates the aforementioned drawback by using two identical resonant inductors. This arrangement results in inductive loads for both switching legs when the operating Manuscript received November 4, 1992; revised February 24, 1993. This work was supported by the National Science Foundation under Grant ECS8922695. This paper was recommended by Associate Editor John Choma, Jr. The authors are with the Department of Electrical Engineering, Wright State University, Dayton, OH 45435. IEEE Log Number 9209003. Fig. I . Single-capacitor phase-controlled Class D series resonant inverter. frequency is higher than the resonant frequency by a factor of 1.15. Therefore, power MOSFET’s without snubbers can be used as power switches. The objective of this paper is to present a new phasecontrolled series resonant converter along with the frequencydomain analysis for steady-state operation, and experimental results. The significance of this work is that it introduces a new efficient constant-frequency resonant converter along with tools for its analysis and design. The phase-controlled Class D inverter shown in Fig. I consists of a dc input voltage source VI , two switching legs, two resonant inductors L, one resonant capacitor C, an ac load R,, and a coupling capacitor Cc. Each switching leg comprises two switches with antiparallel diodes. If the load resistance Ri in the inverter of Fig. 1 is replaced by one of the Class D current driven rectifiers [13] shown in Fig. 2, a single-capacitor phase-controlled series resonant converter (SC PC SRC) is obtained. Its dc output voltage Vo can be regulated against load and line variations by varying the phase shift between the voltages which drive switching legs while the operating frequency is maintained constant. 11. ANALYSIS OF CLASS D PHASE-CONTROLLED INVERTER A. Assumptions The analysis of the phase-controlled Class D inverter of Fig. 1 is performed under the following simplifying assumptions: The loaded quality factor QL of the resonant circuit is high enough that the currents i l and i 2 through the resonant inductors are sinusoidal. The power MOSFET’s are modeled by switches with ON-resistance rDs. The reactive components of the resonant circuit are linear, time invariant, and do not have parasitic resonances. Both resonant inductors are identical. B. Voltage Transfer Function of Phase-Controlled Class D Inverter In Fig. 1, the switching legs and the dc input VI form square-wave voltage sources. Since the currents i l and 22 1057-7122/93$03.00